1. Field of the Invention
This invention relates to an apparatus for generating data-reproducing clock signals, and a data reproducing apparatus. More particularly, it relates to an apparatus for generating data-reproducing clock signals in which outside clock signals are phase-corrected by using reference clock signals recorded on the recording medium, and a data reproducing apparatus in which data are reproduced by using such phase-corrected outside clock signals.
2. Prior Art
In a magnetic field modulation magneto-optical recording system, those portions on a disc which have reached the Curie temperature by laser irradiation are magnetized in accordance with an orientation of an externally applied magnetic field to record data. That is, a laser diode, for example, is driven by pulses to emit light, an external magnetic field is modulated by data to be recorded and pits are thereby formed sequentially on the recording surface. The sizes are determined by the spot size of the laser beam and correspond to the portions of the disc magnetized by the outside magnetic field. If, for raising the recording density, the duration of the pulses which drive the laser light source is selected to be smaller than the spot size of the laser beam, a portion of the laser beam spot overlaps a portion of the previously formed pit so that, as shown in FIG. 1, the newly formed pit overwrites a portion of the previously formed pit. That is, the pits are formed sequentially and an overlapping is produced if the recording density is raised. The result is the so-called edge recording system in which data are carried by transition points or edges of the orientation of magnetization, such as the NRZI (non-return-to-zero-inverted) system.
In a magneto-optical recording system, such as a system with a magneto-optical disc which is rotationally driven by a spindle motor at a constant linear velocity (CLV) or at a constant angular velocity (CAV), pits are arranged as a spiral pit row or as concentric pit rows, respectively. With CAV driving, the distance or length traversed by a recording head during a unit time interval is lesser on the inner periphery than on the outer periphery, so that, as shown in FIG. 2, the amount of overlapping on the inner periphery is larger than on the outer periphery. If data are reproduced from the pits recorded in this manner, the signals reproduced from the pits on the inner periphery are temporally out of phase, that is, advanced or shifted, relative to those reproduced from the pits on the outer periphery, as shown in FIG. 3. If, with the so-called outside clocking system not employing self-clocking, the pits on the inner periphery are reproduced by using the outside clock signals, that is, the synchronizing clock signals from the magneto-optical disc for reproducing the pits on the outer periphery, the above mentioned transition points or edges cannot be located correctly. In other words, the detection window becomes narrower and, if the so-called NRZI modulation system is used for modulating the magnetic field, the noise margin relative to the threshold value for checking if there is or there is not an edge during reproduction is reduced.
For overcoming these deficiencies, corrections may be made during recording on the basis of radius data of the recording disc or ambient recording conditions, such as the temperature of power of the laser light source, before proceeding to the pit forming operation. However, it is difficult to set forth the detailed recording conditions for appropriate corrections. Additionally, since the disc properties differ from one disc to another, and the properties of the data recording apparatus also differ from those of the data reproducing apparatus, it is not sufficient to make corrections at the time of recording. Thus, to achieve a high recording density, it is necessary to correct the signals reproduced from the magneto-optical disc so that they are temporally in phase with each other on the basis of the signals themselves at the time of data reproduction.
While there is known a so-called self-clocking system in which clock signals superimposed on data signals recorded on a magneto-optical disc are extracted during reproduction and used as reproducing clock signals, it is necessary with this system to use a dedicated modulation system and to provide a sufficient clock capturing region to enable correct clock extraction.